This Research award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports work by Professor T. Brent Gunnoe at the University of Virginia to probe the activation of carbon-hydrogen bonds by transition metal complexes. Formally anionic heteroatomic ligands coordinated to high d-electron count transition metals exhibit reactivity patterns consistent with highly basic and nucleophilic character. This feature can potentially be exploited to develop new catalysts for C-H activation and functionalization, and the mechanistic studies of the 1,2-addition of C-H bonds across metal-heteroatom bonds will provide fundamental details that are key to the development of catalysts for hydrocarbon functionalization. The strategy is to build multiple transition metal based platforms upon which to study the C-H activation transformations in an effort to develop an understanding of the impact of metal identity, oxidation state and heteroatom ligand identity on the reaction rates and substrate selectivity. The research program will provide a training ground for undergraduate and graduate students in the areas of organometallic chemistry and development of homogeneous catalysts. In addition, a portion of the proposed research is in collaboration with a PI from a primarily undergraduate institution, thereby providing undergraduates with an opportunity for a productive research experience.
The selective functionalization of hydrocarbons is important to the commodity chemical sector, fine chemical synthesis and the efficient utilization of fossil fuels as energy resources. New methods for selective hetero-functionalization of hydrocarbons can provide important new synthetic paradigms with broad impact.
Hydrocarbons are the primary component of natural gas and petroleum, and greater than 90% of materials made by the petrochemical industry are derived from a few hydrocarbons that are derived from natural gas and petroleum. The rising demand for fossil resources (especially petroleum) has placed a strain on fossil resources. Current catalyst technologies for hydrocarbon functionalization (i.e., the conversion of hydrocarbons into higher value and useful compounds) are often energy intensive and inefficient. New catalyst technologies for the conversion of hydrocarbons into higher value compounds could reduce the expense and environmental impact of hydrocarbon processing. But, the selective functionalization of hydrocarbons stands as one of the foremost challenges for synthetic chemistry, and the develeopment of new catalysts depends on new fundamental advancements. This research project was directed toward new understanding of processes for hydrocarbon functionalization. Two primary research outcomes are described below. 1) Selectively breaking nonpolar bonds (e.g., silicon-hydrogen, hydrogen-hydrogen and carbon-hydrogen bonds) is a central reaction for new catalysts, and understanding how such reactions occur can allow rationale improvements. With combined experimental and theoretical work, we discovered that a platinum complex breaks the hydrogen-hydrogen bond of dihydrogen by a novel pathway. A soluble Pt complex in the presence of dihydrogen forms elemental Pt, which then serves as a catalyst for the cleavage of H-H bonds. This development opens the door to new approaches to break nonpolar bonds and, ultimately, catalysts for hydrocarbon functionalization. 2) The introduction of nitrogen or oxygen based functionality into hydrocarbons is of primary importance. Thus, new methods to form carbon-nitrogen and carbon-oxygen bonds are central to the development of new catalysts for hydrocarbon processing. We developed new copper complexes that catalyze the addition of oxygen-hydrogen and nitrogen-hydrogen bonds across carbon-carbon triple bonds (i.e., alkynes) to form oxygen- or nitrogen-based heterocycles. Such compounds are central units in compounds that have biological activity. Our new catalysts, which are based on earth abundant and inexpensive copper, produce heterocycles that have carbon-carbon double bonds (i.e., olefins) outside the newly formed ring. Catalysts that selectively produce such heterocycles are relatively rare, and previously reported catalysts that achieve the same type and level of selectivity are based on lanthanide complexes, which are highly air sensitive and difficult to handle and store in most cases. In contrast, our copper catalysts are tolerant of air. Given the substantial interest and potential value of catalytic hydroalkoxylation and hydroamination of carbon-carbon multiple bonds, and the inexpensive nature of copper, we believe that this development will generate interest in related catalysts. In addition to research outcomes, the research project provided a training ground for graduate students and postdoctoral researchers. A total of six graduate students and one postdoctoral researcher participated in this project. Two of the six students have completed their Ph.D. degree. One of these students is currently employed by the U.S. Army Research Laboratory and the other is an Assistant Professor. We also collaborated with the research group of Professor Laurel Habgood at Rollins College in Florida. This collaboration augmented the research experience and training for the undergraduate students at Rollins College.